As is well known, in wide band direct sequence CDMA systems signals consist of different pseudo-random binary sequences that modulates the carrier. Thereby, the spectrum of the signals is spread over a wide frequency range common to a number of channels in the system. Due to the direct sequence coding, orthogonality between signals is achieved, enabling individual decoding of signals from the common frequency range.
This coding principle has many advantages. For instance, direct sequence spread spectrum coding provide substantial reductions of the severity of multi-path fading, which leads to an effective utilisation of spectrum resources.
Since signals occupy the same space in the frequency/time domain, power regulation of the individual channels is an important aspect of CDMA systems.
CDMA systems employ power control on both the up- and the downlink. One objective of the power control is to regulate each mobile station transmitter operating within the cell site base-station receiver, such that the signals have the same power level at the base-station receiver, regardless of the position or propagation loss of the respective mobile stations.
When all mobile station transmitters within a cell site are so controlled, then the total signal power at the base-station receiver is equal to the nominal received power times the number of mobile stations.
Each selected signal received at the base-station is converted into a signal that carries the narrowband digital information, whereas the other signals that are not selected remain wide band noise signal. However, the bandwidth reduction, which is performed according to the decoding process, increases the signal-to-noise ratio from a negative value to a level that allows operation with an acceptable bit error rate.
The overall system capacity, for instance the number of users that can operate within the cell simultaneously, depends on the minimum signal-to-noise ratio, which produces the given acceptable bit error rate.
On the downlink, the cell also supports power regulation by adjusting the downlink power for each signal to the respective mobiles in response to requests provided by the mobile station. The purpose is to reduce power for units that are either stationary, relatively close to the cell site, impacted little by multi-path fading and shadowing effects, or experiencing little other cell interference. Thereby the overall noise level diminishes and those mobiles being in a more difficult environment will benefit.
In a CDMA system running close to its capacity maximum, the emission of excessive power on only a single coding channel is critical. A single excessively “loud” mobile will disturb other mobiles, which in their turn will require higher power because of the reduced signal to noise ratio. This effect will spread to adjacent parts of the system and communications in the cell. In a fully loaded cell, the addition of one more mobile will result in about 35% loss of capacity.
Therefore, both up- and downlink channels are minutely regulated to avoid the above overload situation from occurring and for this reason new mobiles may be exempt from being allowed into the system when the system is running close to its capacity maximum.
Current CDMA systems are able to offer a variety of services, such as providing data-communication channels with varying data rates. Since a higher bit error rate corresponds to a higher capacity, also the data rates allocated to individual mobiles may be regulated in order to avoid the above mentioned overload situations from occurring.
Another important aspect of CDMA systems is that the so-called soft hand-over between base stations and inter-cell channels may be implemented.
In CDMA systems, the same channel is re-used in adjacent cells whereby mobiles may receive a combination of the respective same signals being emitted from more base-stations (i.e. neighbouring cells).
In CDMA systems, each cell transmits a pilot carrier signal. This pilot is used by the mobile station to obtain initial system synchronisation and to provide robust, time, frequency and phase tracking of the signals from the cell site. The pilot signal is tracked continuously by each mobile station. Variations in the transmitted power level of the pilot signal control the coverage area of the cell and the number of mobiles communicating with the respective cell.
At call initiation, the subscriber is supplied a tailored set of handoff thresholds and a list of cells that are most likely to be the candidates for hand-off. While tracking the signal from the original cell, the subscriber searches for all possible pilots and maintains a list of all pilots whose signals are above a threshold established in the initial set-up. This list is transmitted to the base-station controller whenever it is requested, whenever the list changes by having a new pilot in the list or when an existing pilot falls below a level that is useful to support traffic.
Upon command from the radio network controller (RNC), via the initial cell, the mobile unit commences tracking the second cell and uses diversity combining of the two signals, representing identical data, to enhance the overall received signal. Power control information is received from both cells; and both cells have to request a power increase for the subscriber to increase its power.
Data from the mobile unit is received by both cells in question and is forwarded to the radio network controller where the best source is selected on a frame by frame basis and is used to represent the data transmitted from the mobile.
A particular advantage by the power regulation described above and the soft-handover is that the capacity of cells is automatically regulated in case of an inhomogeneous cell load.
If some cells are more heavily loaded than others, then the remaining cells contribute less interference to their more heavily loaded neighbours and allow more mobile stations to operate in these cells. The reduction of interference leads to a capacity increase for the loaded cell. For instance, if the surrounding cells run at 30% load, the capacity of the loaded cells increases to 120%.
This flexible allocation of capacity happens automatically due to the power regulation routines described above. The network management system detects the current load situation and allows higher power levels in the heavily loaded cell.
Aspects of the CDMA features described above has for instance been described in “An overview of the application of code division multiple access (CDMA) to digital cellular systems and personal cellular networks”, May 21, 1992, Document Number EX60-10010 Qualcom Incorporated™.
Further details relating to soft handover can for instance be found in EP-A-0 537 795.
Details relating to CDMA systems can be found in the standard denoted 3GPP UTRAN Release 99.